(a) Field of the Invention
This invention relates to an antilock braking method and apparatus for a four-wheel drive vehicle, in which a power unit is connected to one of the front and rear axles, the other axle being connected to the power unit via a torque transmission whose rate of torque transmission increases in accordance with an increase in the relative rotational speed of the axles.
(b) Description of the Prior Art
In a conventional four-wheel drive vehicle, brakes are generally provided on all the wheels mounted on the front and rear axles.
In a vehicle in which one of the front and rear axles is driven by a power unit while the other axle is driven therefrom via a torque transmission, the axles are connected to each other substantially rigidly at all times, so that the axles interfere with each other. Namely, if the brakes are applied to the wheels on one of the axles, especially the front axle which supports a larger load, this has the effect that the brakes are applied to the wheels on both the front and rear axles at once. In the case where the hydraulic braking pressure becomes excessively high when the above-described vehicle having brakes on all the wheels is suddenly braked, all the wheels are locked at once. Therefore, the stability of this vehicle during a braking operation is improved as compared with that of a front wheel drive vehicle or a rear wheel drive vehicle but the problems of a lack of stability with respect to disturbance and a loss of steering capability of the four wheel drive vehicle still remain unsolved. Consequently, it is desirable that the brake system be furnished with antilocking functions.
However, an antilock brake system is basically designed to control the braking torque for each wheel or axle individually and thereby retain a slip ratio for each wheel at a proper level, and it is necessary that the speed of each wheel or axle be varied irrespective of one another. On the other hand, in a vehicle in which the front and rear axles are connected substantially rigidly, these axles interfere or interact with each other. Therefore, it is impossible to provide suitable antilock control for each brake in a conventional vehicle which is provided with a brake on each wheel.
In a vehicle in which the front and rear axles are connected substantially rigidly, upon setting the wheels on one axle with a suitable slip ratio this necessarily causes the slip ratio of the wheels on the other axle to attain substantially the same level.